The defective interfering (DI) particle generated by a heat resistant mutant HR of VSV is capable to transcribe its own genome into biologically active mRNA. It is also capable to interfere with virion primary transcription in mixedly infected cells. We have prepared HR DI particles temperature sensitive (ts) in self transcription by growing the wild type HR DI particle in the presence of a helper virion which was ts in its L protein (transcriptase). This ts HR DI particle was used in mixed infections with wild type VSV and its ability to interfere with virion primary transcription and replication at permissive and nonpermissive temperatures was determined. The results indicated that self transcription of the HR DI particle was mandatory for both types of interference with viral functions. On the other hand replication of the ts HR DI particle in the presence of the helper virus took place even without self transcription and interference by this particle. Our previous work on the effect of adenosine on VSV replication indicated that the presence of adenosine modified the replicative enzyme and that this modification resulted in increased transcription. We have now isolated adenosine resistant mutants of VSV. These mutants exhibited normal levels of transcription in the presence of adenosine and suppressed levels of transcription in its absence. An analysis of the virion proteins by partial proteolysis digestion demonstrated that the NS protein was altered in the mutant virions.